1. Field of the Invention
This invention relates to a filter for use in cleaning air and collecting or removing dusts contained in the air for filtering and collecting the dusts to separate them from the air.
2. Description of the Prior Art
There are widely used, as a filter, those manufactured by bending a sheet (monolayer sheet) or a composite sheet composed of a laminate of at least two said monolayer sheets in the form of a pleat, which laminating sheet is made of various fiber materials such as same or different natural fibers, synthetic fibers, and glass fibers.
The above filter is designed for filtering and collecting fine dusts contained in the air due to mechanical functions, such as inertia, diffusion, gravity and shut-off by passing the air through the filter in a direction substantially perpendicular to the filter material.
In the case of an electret (dielectric) filter, the filtering and collecting are further due to electrostatic attraction to the dusts. In general, the composite sheet is often used as a filter material.
Various composing techniques are applied for laminating the monolayer sheets with each other. These techniques include, for example, a laminating method with an adhesive, an ultrasonic or a fusion welding method using a thermo-embossing machine or an extreme infrared radiation heater.
With the composite sheet made in a pleat-like shape, it is possible to make a filter having relatively larger area for filtration. In addition, the pleat filter has less air-permeating resistance and can improve dust-collecting capacity and lifetime. On the contrary, a conventional filter material composed of a composite sheet, which is manufactured by laminating monolayer sheets with an ultrasonic or a fusion welding, is fusion-welded over at least a part of the filter material and the welded portion and the adjacent portion thereto are not a fiber-like or has a fiber structure of usually high density, resulting in decreasing the effective filtration area of the filter material and in losing a loft to thereby affect unfavorably on filtering properties such as air-permeating resistance and lifetime. Particularly, the filtering properties of the electret filter are more unfavorably affected by the conventional method since, in the case of composing the monolayer sheets by the fusion welding method to laminate the same, the electric charge of the electret of the filter material is reduced due to the heat of the fusion. The electret charge (the electric charge of the electret) may be extinguished depending on the extent of the heating temperature or heating time or the synergistic effect thereof. For example, the electret charge of a composite sheet composed of a non-woven fabric sheet, which is formed from a normal resin, is substantially extinguished at the welded portion thereof.
In the case of composing the monolayer sheets using an adhesive to laminate the same, the air permeability of the filter is reduced at an adhesive-boned portion, resulting in the deterioration of the filtering properties. Further, in the electret filter, the electret charge is neutralized with the adhesive itself or a solvent contained therein to be reduced or extinguished.
Additionally, the conventional method for manufacturing such filters needs a process for laminating monolayer sheets prior to a process for bending a composite sheet to pleat the same.